1. Field of the Invention
The present invention relates to a control device for a heater for an air fuel ratio sensor which is located in an intake air passage of an internal combustion engine.
2. Description of the Related Art
An engine control device which determines engine control parameters based on an oxygen concentration of intake air detected by an air fuel ratio sensor located in an intake passage has been disclosed. In Japanese Unexamined Patent Publication No. 62-78469, an air fuel ratio sensor which is provided with a heater for stabilizing an output of the air fuel ratio sensor is disclosed.
In the above type heater provided sensor, electric power supply to the heater is determined based on the amount of intake air. However, the air fuel ratio sensor is affected not only by an amount of intake air, but also by the temperature of ambient air around the sensor. For example, in the case that the engine has an EGR system which recirculates a part of exhaust gas to the intake passage upstream of the air fuel ratio sensor, the ambient air around the sensor is affected by the recirculated exhaust gas. Therefore, a system which determines the electric power supply for a heater based only on an amount of an intake air cannot always supply the required power appropriately. If excessive power is supplied, heater will be overheated and may be broken. If too little power is supplied, the sensor will not produce a proper output.